The present invention relates to a tire tread band for medium/heavy duty motor vehicles, comprising at least one pair of circumferential shoulder ribs confined between respective side edges of the tread band and respective circumferential shoulder grooves axially spaced apart from the side edges. At least one pair of intermediate circumferential ribs is respectively confined between the shoulder grooves and corresponding intermediate circumferential grooves axially spaced apart from the shoulder grooves. At least one center rib extends circumferentially between said intermediate grooves, each of said intermediate and center ribs are divided into a plurality of respectively intermediate and center raised reliefs or blocks, which are circumferentially defined by respectively intermediate and central transverse cuts of a width lower than 1 mm.
It is known that medium/heavy duty motor vehicles, such as trucks, semi-trailers and the like are usually equipped with tires of different types on the driving and driven axles respectively, due to the different requisite behavior and operation qualities of the tires, taking into account the different conditions of use.
In particular, tires associated with the driving wheels must exhibit as the fundamental quality, a high traction capability on any type of road, in addition to excellent uneven wear resistance for achieving high tire mileage. To this end, tires are used the tread band of which is of the type provided with a plurality of blocks defined by the combination of circumferential grooves intersecting transverse grooves of a defined width, usually in the order of 5 mm, oriented according to an angle generally larger than 45.degree. with respect to the direction of the circumferential extension thereof. The sizes and orientation of the transverse grooves greatly affect the grip action exerted by the tire on the ground in relation to the tangential efforts connected with the transmission of the motive force.
The state of the art also suggests different construction expedients to be employed on driving or tractive tires mainly in cars, for the purpose of improving the operating features thereof. For example, European Patent EP 0 498 287 discloses a tread band provided with five rows of blocks, to be respectively identified as a center row disposed on the equatorial plane of the tire, two shoulder rows disposed along the opposite side edges of the tread band, and two intermediate rows, one being interposed between a center row and each of the shoulder rows. For the purpose of improving the wear evenness of the blocks, the transverse grooves associated with the shoulder and center block rows have a given angle of incidence relative to a direction normal to the tread band surface, which angle of incidence has respectively contrary directions between the grooves of the shoulder rows and those of the center row. It is also pointed out that two opposite types of rotation are provided for in such a tread band, depending on whether the corresponding tire is mounted to a traction axle or a driven axle of the motor-vehicle, taking into account the opposite orientations of the reaction forces exchanged with the ground in the two different use situations.
Patent GB 2 093 777 discloses a tire for cars, particularly adapted to run on road-beds covered with snow, the tread band of which is divided into five circumferential ribs, identified as a center rib, a pair of intermediate ribs and a pair of shoulder ribs, integrally passed through by thin grooves and transverse cuts oriented obliquely to the circumferential direction. The grooves and cuts arranged in the intermediate and shoulder ribs have a predetermined angle of incidence relative to the normal to the external surface of the tread band, for the purpose of improving the grip action of the tread band on the road-bed when covered with snow. In order that the rolling direction of the tire will not affect the traction features thereof, it is provided that the incidence of the grooves and cuts made in one of the shoulder ribs and one of the intermediate ribs be opposite from that of the grooves and cuts belonging to the other shoulder rib and other intermediate rib.
Referring now to tires mounted on the driven axles of medium/heavy duty motor-vehicles, it is noted that the main feature required from them is a high directional control stability, together with a good abrasion resistance and wear unevenness resistance, as well as a remarkable elimination of water from the imprint or track area in case of running on a wet road-bed.
In this connection tires having a tread band of the "furrowed" type are used, that is provided with circumferential straight or zigzag grooves defining a plurality of circumferential ribs in the tread band, which ribs are optionally passed through by transverse cuts of limited width.
European Patent EP 0 384 182 discloses a tire for medium/heavy duty motor-vehicles having a tread band of the "furrowed" type, with five or more circumferential ribs confined by corresponding circumferential zigzag grooves. Each rib, apart from the shoulder ribs defining the opposite side edges of the tread band, is divided into a plurality of blocks circumferentially defined by transverse cuts less than 3 mm wide. The transverse cuts of each rib are located in a circumferentially offset position relative to those of the adjacent rib and have an angle of incidence between 50 and 25.degree. relative to a direction normal to the external tread band surface. The rotational direction of the tire is such that on running the reactions transmitted from the ground to the ground-contacting area of the tread band tend to reduce the inclination of the transverse cuts relative to the normal to the tread band surface, for the purpose of achieving a greater wear evenness in the blocks, thereby avoiding the so-called "saw-tooth" wear phenomenon.
The foregoing being stated, due to the fact that at the present state of the art the tread band pattern adopted for traction tires and driven or free rolling tires respectively is specific for each of them, the use of one and the same type of tire for both uses on a medium/heavy duty motor-vehicle cannot be proposed.